This present invention relates to antenna systems which may be used on vehicles to communicate with both a satellite and a terrestrial system.
There is currently a need for antennas and/or antenna systems that can communicate with both a satellite and a terrestrial system. One example of such a need is for a Direct Broadcast Satellite (DBS) radio in which radio signals are broadcast from a satellite and are received by a receiver located on vehicle and also received by terrestrial repeaters which rebroadcast the signals therefrom to the same vehicle. Typically, a direct broadcast satellite uses circular polarization so that the vehicle can receive the transmission in any orientation. However, terrestrial networks typically transmit in vertical polarization. If satellite communication fails (for example, if the satellite becomes hidden by a building or other objectxe2x80x94manmade or natural) the terrestrially rebroadcast signal can be used to fill in the gaps in the satellite signal.
DBS radio systems typically have a narrow bandwidth (about 0.5%) due to the low power available from satellites, as well as the problems associated with mobile wireless communications. On the other hand, an antenna typically must be designed with at least several percent bandwidth to account for possible errors in manufacturing. For this reason, the antennas used to receive DBS radio signals will generally have a much wider bandwidth than the signals of interest (both satellite and terrestrial), and thus the various components of DBS signals can be considered as being essentially at the same frequency.
There is a need for antennas or antenna systems that can receive radio frequency signals having circular polarization and/or linear vertical polarization. Furthermore, the antenna or antenna system should preferably be able to utilize different radiation patterns for each of these two functions. The antenna or antenna system should have a radiation pattern lobe with circular polarization directed towards the sky, at the required elevation angle for satellite reception, and also have a radiation pattern lobe with linear polarization directed towards the horizon, for terrestrial repeater reception.
Antennas exist that can perform these two functions. For example, a quadrafilar helix antenna, which consists of four wires wound in a helical geometry, can do so. The drawback of this antenna is that it typically protrudes one-quarter to one-half wavelength from the surface of where ever it is mounted and thus if it is mounted protruding from the exterior surface of a vehicle, it results in an unsightly and unaerodynamic vertical structure.
The antenna disclosed herein performs these two functions yet lies essentially flush with the roof of the vehicle. It is able to perform as a dual circular/linear antenna, with the ability to form beams in various directions. It has the added advantage that it can incorporate beam-switched diversity for an improved signal to noise and interference ratio.
This invention improves upon the existing vertical rod antenna that is currently used for satellite and terrestrial radio broadcasts. The disclosed antenna is much less than one-tenth of one wavelength in thickness, and can be placed directly on a metal vehicle roof and lies flush or essentially flush therewith.
The present invention utilizes a Hi-Z surface, a particular kind of ground plane that has been demonstrated to be useful with certain low-profile antennas. The present invention preferably uses four linear wire antenna elements arranged a radial pattern, the four wire antennas being fed by a beam forming network that generates the desired polarizations and beam patterns. Other antenna elements can alternatively be used. The beam forming network has two or more outputs that are routed to a radio receiver, for example (a transceiver could be used if the antenna system is used for both receiving and transmitting signals). The antenna disclosed herein also provides the option for beam switched diversity, providing even better performance. The primary advantage of this antenna is that it is thin, and can be mounted directly on or concealed within the metal roof, for example, of a vehicle.
The prior art includes:
(1) D. Sievenpiper and E. Yablonovitch, xe2x80x9cCircuit and Method for Eliminating Surface Currents on Metalsxe2x80x9d U.S. provisional patent application, serial number 60/079953, filed on Mar. 30, 1998 by UCLA and corresponding PCT application PCT/US99/06884, published as W099/50929 on Oct. 7, 1999, the disclosures of which are hereby incorporated herein by reference.
(2) U.S. Pat. No. 5,929,819, xe2x80x9cFlat antenna for satellite communicationxe2x80x9d, by Grinberg, Jan and assigned to Hughes Electronics Corporation. While this patent describes a flat antenna for satellite reception, it is not nearly as flat as the present invention, because it requires elevated lenses. Furthermore, it does not provide for also communicating with a terrestrial system.
(3) U.S. Pat. No. 6,005,521, xe2x80x9cComposite antennaxe2x80x9d, by Suguro, Akihiro and Ookita, Hideto, which patent was assigned to Kyocera Corporation. The antenna disclosed therein provides for diversity reception of signals having different polarizations. However, it is well suited for integrating into a vehicle because of the requirement for a section having a vertical projection.
(4) U.S. Pat. No. 6,081,239, xe2x80x9cPlanar antenna including a superstrate lens having an effective dielectric constantxe2x80x9d, by Sabet, Kazem F.; Sarabandi, Kamal; and Katehi, Linda P. B., which patent was assigned to Gradient Technologies, LLC. This patent describes various ways of making a lens having an effective dielectric constant, and the combination of that lens with an antenna. This disclosed concept can be employed with the present invention to control the radiation pattern of the disclosed antenna.
(5) R. Vaughan, xe2x80x9cSpaced Directive Antennas for Mobile Communications by the Fourier Transform Methodxe2x80x9d, IEEE Transactions on Antennas and Propagation, vol. 48, no. 7, pp. 1025-1032, July 2000.
(6) P. Perini, C. Holloway, xe2x80x9cAngle and Space Diversity Comparisons in Different Mobile Radio Environmentsxe2x80x9d, IEEE Transactions on Antennas and Propagation, vol. 46, no. 6, pp 764-775, June 1998.
(7) C. Balanis, Antenna Theory, Analvsis, and Design, 2nd edition, John Wiley and Sons, New York, 1997.
Related applications include the following:
(1) D. Sievenpiper, J. Schaffner, xe2x80x9cA Textured Surface Having High Electromagnetic Impedance in Multiple Frequency Bandsxe2x80x9d, U.S. patent application Ser No. 09/713,117 filed Nov. 14, 2000.
(2) D. Sievenpiper, H. P. Hsu, G. Tangonan, xe2x80x9cPlanar Antenna with Switched Beam Diversity for Interference Reduction in Mobile Environmentxe2x80x9d, U.S. patent application Ser. No. 09/525,831 filed Mar. 15, 2000.
(3) D. Sievenpiper; J. Schaffner; H. P. Hsu; and G. Tangonan, xe2x80x9cA Method of Providing Increased Low-Angle Radiation Sensitivity in an Antenna and an Antenna having Increased Low-Angle Radiation Sensitivityxe2x80x9d, U.S. patent application Ser. No. 09/905,796 filed on the same date as this application.
In one aspect, the present invention provides an antenna for receiving circularly polarized signal from a position relatively high in the sky and at the same time linearly polarized signals from a position relatively lower in the sky and closer to the horizon, the antenna comprising a high impedance surface and a plurality of antenna elements disposed on said high impedance surface and arranged in a pattern on said surface, first selected ones of said antenna elements being responsive to circular polarization and second selected ones of said antenna elements being responsive to linear polarization.
In another aspect, the present invention provides a method of receiving circularly polarized signal from a position relatively high in the sky and at the same time linearly polarized signals from a position relatively lower in the sky and closer to the horizon, the method comprising the steps of: providing a high impedance surface; and disposing a plurality of antenna elements on said high impedance surface and arranging the plurality antenna elements in a pattern on said surface such that first selected ones of said antenna elements are responsive to circular polarization and second selected ones of said antenna elements are responsive to linear polarization.
In yet another aspect, the present invention provides an antenna system for receiving both circularly polarized radio frequency signals and linearly polarized radio frequency signals, the circularly polarized signals arriving at the antenna system from a direction normal or oblique to a major surface of the antenna system and the linearly polarized signals arriving at the planar antenna system from a direction acute to said major surface, the antenna system comprising a high impedance surface and a plurality of antenna elements disposed on said high impedance surface, the plurality antenna elements arranged in a pattern on said surface such that first selected ones of said antenna elements are responsive to circular polarization and second selected ones of said antenna elements are responsive to linear polarization.